Water (mix)

• Water, with the molecular formula H2O, consists of two hydrogen atoms covalently bonded to an oxygen atom
• Liquid water is essential for life, and composes over 70% of a cell's cytosol, allowing solutes to dissolve and interact
• Water is a critical metabolite involved in hydrolysis, photosynthesis, and cell respiration, and acts as a temperature buffer to maintain a stable cellular environment

Polarity

• In water, electrons are not shared equally between atoms
• This unequal sharing leads to a slight charge difference, creating polarity
• Oxygen is more electronegative, attracting electrons more strongly than hydrogen, resulting in oxygen having a slightly negative charge (δ-) and hydrogen having a slightly positive charge (δ+)

Hydrogen Bonding

• Hydrogen bonds are polar associations that form between a δ+ hydrogen atom and a δ- fluorine, oxygen, or nitrogen atom
• In water, hydrogen bonding occurs between oxygen and hydrogen dipoles of different water molecules
• Hydrogen bonds are constantly forming, breaking, and reforming due to molecular motion

Properties of Water

• Water's unique properties, arising from its polarity and hydrogen bonding, contribute to its role as the medium for life
• Water has solvent properties and can dissolve charged and polar substances, as well as cohesive and adhesive properties, enabling it to form associations with other water molecules
• Water has unique thermal and physical properties that contribute towards its buoyancy and viscosity

Solvent Properties

• Water is known as the universal solvent because of its capacity to dissolve a large number of substances
• It can associate with charged (ions) or electronegative (polar) substances due to its polarity
• Polar regions of water molecules form hydration shells around molecular surfaces with opposing charges
• Substances soluble in water are hydrophilic ('water-loving'), while those insoluble are hydrophobic ('water-hating')
• Water's capacity to dissolve substances makes it a medium for metabolic reactions in cells
• Water acts as a reagent or product in metabolic processes such as condensation and hydrolysis

Cohesive Properties

• Water molecules stick together through intermolecular hydrogen bonds, resulting in high surface tension
• At the fluid surface, water molecules form hydrogen bonds with internal molecules, creating an inward force that causes the surface to cohere
• This cohesion allows small insects to glide on the surface of water

Adhesive Properties

• Water can stick to other molecules with similar properties (ionic or polar)
• Water's adhesive properties cause it to form a meniscus in a glass measuring cylinder, as it adheres to the cylinder's sides
• Capillary action, the attraction of water to charged or polar surfaces, allows it to flow against gravity when under tension
• It depends on the diameter of the pore the water is moving through (smaller diameter = more capillary action)
• This allows water to be transported up plant stems via xylem vessels
• The cell walls of xylem vessels contain cellulose, which is a polar polysaccharide and allows water molecules to adhere to the walls

Thermal Properties

• Water can absorb significant thermal energy (heat) before changing state because extensive hydrogen bonding between water molecules must be disrupted before it changes state, requiring heat absorption
• Water has high melting (0°C) and boiling points (100°C) compared to similar liquids
• A high specific heat capacity is defined as the energy needed to heat 1 gram of a substance by 1°C
• Water requires a high heat of vaporization (energy absorbed per gram to change from a liquid to a gas)
• It also has a high heat of fusion (amount of energy lost when 1g of liquid becomes 1g of solid at 0°C)
• Being slow to change temperature, allows water to support is useful for maintaining constant conditions
• Water makes an excellent coolant as a result of its thermal properties
• Evaporation of sweat cools organisms as body heat is used to convert sweat into vapor

Physical Properties

• Polarity promotes both buoyancy and viscosity
• Buoyancy, the upward force on an object, depends on the density of the medium
• Water is denser than organic solvents and air, increasing buoyancy
• Salt water, due to forming a solution, is more buoyant than fresh water, so organisms can float because of water's bouyancy
• Hydrogen bonding makes water more viscous (resistant to flow)
• Viscosity of blood plasma is increased by the presence of cells and dissolved solutes

Water Versus Air

• Physical and thermal properties of water affect survival
• Water and air present different survival challenges
• Relative to air, water has higher thermal conductivity, specific heat capacity, buoyancy and viscosity

Aquatic Adaptations

• Black-throated loons (Gavia arctica) and ringed seals (Pusa hispida) adapted to interact with both water and air in the Arctic

Black-Throated Loon Adaptations

• Solid bones allow the loon to dive underwater for prey while retaining buoyancy
• Legs are located at the rear, making it effective at propulsion in water (more viscous)
• Feathers form an interlocking structure to prevent heat loss in water

Ringed Seal Adaptations

• Denser bones allow it to stay submerged longer (less buoyancy)
• Seals have streamlined bodies to move efficiently through water (higher viscosity than air)
• An outer coat of fur traps air for waterproofing, and a thick layer of blubber prevents heat loss in water
• Ringed seals do not possess many effective physiological cooling mechanisms because water temperatures are stable